The market for semiconductor transistors is becoming increasingly competitive as more manufacturers are introducing a wider variety of semiconductor transistors. As a result, consumers have more products from which to choose and, accordingly, must make more decisions as to what transistor to purchase from which manufacturer. Perhaps one of the most important factors influencing consumers' purchasing decisions is the reliability of a transistor over its expected life. Recognizing the significance that a transistor's reliability may have in the marketplace, manufacturers employ various tests to ensure that each of their respective transistors will continue to meet its original specifications even after prolonged use. Manufacturers usually publish these specifications in a transistor's data sheets and guarantee that each transistor meets its published specifications and will continue to do so even after prolonged use.
One well known method of testing the reliability of a particular transistor is to measure the transistor's specifications immediately after fabrication and then, after subjecting the transistor to a burn-in process, again measure the transistor's specifications to determine if the specifications measured after burn-in are consistent with the original specifications. In performing such a test, manufacturers typically measure various combinations, or even all, of the following specifications for a given transistor: (1) the forward bias voltage for the source-drain junction (V.sub.SD) , (2) the leakage current between gate and source ( I.sub.GSS) , (3) the threshold voltage (V.sub.TH), (4) the leakage current between drain and source (I.sub.DSX) , (5) the transistor's on-resistance (R.sub.DS), and (6) the transistor's breakdown voltage (V.sub.BV). These specifications may be measured using, for example, a FETTEST series 3600 or series 9400 menu-driven die tester.
After the original specifications are measured, the transistor is placed into a furnace and heated to approximately 150 degrees Celsius or higher for 160 to 170 hours. This time-consuming burn-in process simulates the aging process of the transistor. The burned-in transistor is then removed from the furnace and its specifications are measured again as described above. A transistor which still meets its original specifications after being subjected to the burn-in process passes the test and is accordingly deemed to be reliable. Thus, the burn-in process is used to determine whether a transistor may have long-term reliability problems.
The burn-in test may be conducted on every transistor or just on a sampling of the transistors. Trench transistors have become prominent in power applications due to their high voltage and high current characteristics. The trench transistor utilizes a vertical structure wherein a polysilicon gate is formed in a trench etched in the transistor's substrate as described in U.S. Pat. No. 5,072,266 issued to Bulucea et al, incorporated herein by reference. FIGS. 1A and 1B show the formation of such a prior art trench transistor 10. Trench transistor 10 has N+ layer 12 which acts as a drain, N- layer 13 overlying drain 12, P+ layer 14 overlying layer 13, and N layer 16 overlying layer 14. Layer 16 serves as a source for transistor 10, while layer 14 acts as a body. Layer 13 acts as a drift region for transistor 10. Trench 18 is etched in transistor 10 so as to intersect drift region 13, body layer 14, and source 16, as shown in FIG. 1A.
Those surfaces of layers 13, 14, 16 created during the etching of trench 18 will be collectively denoted as trench-surface 19, as shown in FIG. 1B. A layer of oxide 20 is then formed on trench-surface 19 of transistor 10. Polysilicon gate 22 is deposited in trench 18 such that gate 22 is insulated from drift region 13, body layer 14, and source 16 by oxide 20. The structure of trench transistor 10 results in a more efficient utilization of semiconductor surface area, thereby saving space and cost.
The unique structure of the trench transistor, however, leads to problems for testing the transistor's long term reliability. Referring again to FIG. 1B, many defects may arise during the etching of trench 18 which affect the quality of the surfaces of layers 13, 14, 16 created during the etching of trench 18. These defects may include, for example, non-uniform surface conditions of trench-surface 19 and oxide 20, stress-induced defects between gate 22 and oxide 20 and between oxide 20 and trench-surface 19, defects within drift region 13, dangling bonds, and impurities present near the oxide 20/trench-surface 19 interface. These defects, being somewhat unique to trench transistors, may not be effectively detected by the conventional functional or parametric electric test which, as described above, was developed for testing planar and other non-trench transistors.
Accordingly, there is a need for a better, more accurate method for detecting defects in trench transistors. There is also a need for a such a method which is not as time consuming as the burn-in process discussed above.